Earthquakes hit urban centres in Europe infrequently, but occasionally with disastrous effects. Obtaining an unbiased view of seismic hazard (and risk) is therefore very important. In principle, the best way to test probabilistic seismic hazard assessments (PSHAs) is to compare them with observations that are entirely independent of the procedure used to produce PSHA models. Arguably, the most valuable information in this context should be information on long-term hazard, namely maximum intensities (or magnitudes) occurring over time intervals that are at least as long as a seismic cycle. The new observations can provide information of maximum intensity (or magnitude) for long timescale as an input data for PSHA studies as well. Long-term information can be gained from intact stalagmites in natural caves. These formations survived all earthquakes that have occurred over thousands of years, depending on the age of the stalagmite. Their ‘survival’ requires that the horizontal ground acceleration (HGA) has never exceeded a certain critical value within that time period. Here, we present such a stalagmite-based case study from the Little Carpathians of Slovakia. A specially shaped, intact and vulnerable stalagmite in the Plavecká priepast cave was examined in 2013. This stalagmite is suitable for estimating the upper limit of horizontal peak ground acceleration generated by prehistoric earthquakes. The critical HGA values as a function of time going back into the past determined from the stalagmite that we investigated are presented. For example, at the time of Jókő event (1906), the critical HGA value cannot have been higher than 1 and 1.3 m/s2 at the time of the assumed Carnuntum event (∼340 AD), and 3000 years ago, it must have been lower than 1.7 m/s2. We claimed that the effect of Jókő earthquake (1906) on the location of the Plavecká priepast cave is consistent with the critical HGA value provided by the stalagmite we investigated. The approach used in this study yields significant new constraints on the seismic hazard, as tectonic structures close to Plavecká priepast cave did not generate strong earthquakes in the last few thousand years. The results of this study are highly relevant given that the two capitals, Vienna and Bratislava, are located within 40 and 70 km of the cave, respectively.
Bibliographical noteFunding Information:
Open access funding provided by University of Vienna. The in situ measurements in the Plaveck? priepast cave were supported by the ?sterreichischer Austauschdienst (No. ICM-2012-00497) and the Department of Meteorology and Geophysics, University of Vienna. The authors wish to thank Ferenc Kov?cs, Pavel Stanik (Slovak Cave Administration), Peter Magdolen and Marek Velsmid for professional support in the cave. We are also grateful to Kamil Soucek for the tomographic image of a broken stalagmite from the Plaveck? priepast cave. Some of the figures were prepared using General Mapping Tools (Wessel and Smith 1998) and OBSPY (Beyreuther et al. 2010). The paper was written within the scope of a project of the Institute of Clean Technologies for Mining and Utilization of Raw Materials for Energy Use?Sustainability Programme, reg. no. LO1406, which is supported by the Research and Development for Innovations Operational Programme financed by the Structural Funds of the European Union and the State Budget of the Czech Republic. We also acknowledge the suggestions of Csaba Kegyes-Brassai about the theoretical calculations and the meaning of results of the eigenfrequency and harmonic oscillations of the stalagmite we investigated, the remarks by Tibor Zs?ros about the macroseismic effects of the J?k? (Dobra Voda) earthquake (10 January 1906), the collection of publications about the Plaveck? priepast cave in Slovak language and the translation by Zolt?n Jerg. This paper has improved considerably after thorough reviews of an anonymous reviewer, and we are grateful for his helpful comments, his time and efforts.
- Cantilever beam
- Natural frequency
- Peak ground acceleration
- Prehistoric earthquake
- Seismic hazard